1. Field of the Invention
This invention relates to the packaging of electronic components using lead-free structures and materials.
2. Description of the Related Art
In the electronics industry, it is often necessary to attach chip packages and bare die to printed circuit boards (PCBs). The attachment structures provide electrical connections as well as mechanical support. Thermal management is also an issue in these systems. Circuit boards are often constructed of organic materials with relatively high coefficients of thermal expansion (CTEs), while ceramic chip carriers have relatively low CTEs. Thermal expansion mismatch can cause mechanical reliability problems such as metal fatigue. For some applications the thermal mismatch can be accommodated more easily using column grid array attachment techniques than using ball grid arrays.
In the past, many attachment structures used lead-containing solders. More recently it has been recognized as important to reduce the quantity of hazardous material used in many industries, including the electronics industry. Solders based on alloys of tin, silver, and copper have found use as lead-tin substitutes. The system is often referred to as “S-A-C” after the first letters of the chemical symbols for the constituents: Sn (tin)-Ag (silver)-Cu (copper). NEMI has, in fact, promulgated proposed standard solder formulations, including binary compositions of 0.7% Cu in Sn and 3.5% Ag in Sn for wave soldering, and a ternary alloy containing 3.9% Ag and 0.6% Cu in Sn for solder reflow. It is customary in this art to cite composition by weight or mass, so that 100 grams of the ternary reflow solder contain 95.5 g Sn, 3.9 g Ag, and 0.6 g Cu. The S-A-C eutectic composition of 95.42% Sn, 3.73% Ag, and 0.85% Cu, as reported by NIST, has a melting point of approximately 216° C.